This invention relates to electrical conductive foam beads and molded electrical conductive foamed articles obtained therefrom. More particularly, this invention relates to electrical conductive foam beads of a synthetic resin and molded articles obtained therefrom both having an electrical conductive surface and adapted for use in various applications as electrical conductive cushioning materials, electro-magnetic radiation shields and the like with the advantage of preventing electric injury due to electrostatic discharges.
In the industrial packaging field, foamed plastics such as foamed polystyrene, foamed polyethylene, and foamed polypropylene have been widely used as packaging materials for transportation of a variety of products. Although such foamed plastics are preferred in view of light weight, cushioning properties, and low cost, they undesirably tend to accumulate electrostatic charges in the process of packaging and transportation. On the other hand, household electric appliances such as audio components, video tape decks and microwave ovens, various measuring instruments, medical instruments and other electric products have generally improved their performance by incorporating semiconductor integrated circuits therein. Inconveniently, integrated circuits are liable to failure due to electrostatic charges. When the above-mentioned products are packaged in foamed plastics for transportation, there is the likelihood of electrostatic failure of such products, more specifically electrostatic failure of built-in IC's.
In view of the problem that electrostatic charge causes failure or injury of integrated circuits during storage or upon transportation of integrated circuits and finished products having the same built therein, antistatic or electrical conductive plastics have been developed in order to protect integrated circuits from electrostatic failure. Carbon-containing plastics are generally known as a typical conductive plastics. Particularly, examples of conductive plastics having cushioning properties include (1) rigid or flexible foamed polyurethane having carbon blended therein, (2) rigid or flexible foamed polyurethane impregnated with carbon, (3) foamed polyethylene and polystyrene sheets having a carbon-blended plastic film adhered to or a carbon-containing paint coated on the surface thereof, and the like.
These foamed plastics having carbon blended or impregnated have, however, the problem that carbon particles are not always continuous to each other, resulting in a relatively high resistivity. A problem also arises from partially conductive packaging materials in the form of foamed sheets having a conductive film adhered to either surface or a carbon-containing coating applied to either surface.
Since the foamed sheet interposed between conductive layers is highly dielectric, no electrical conduction takes place between the opposed conductive layers. The electrical conductivity of such packaging materials is limited to their surface portions so that sufficient antistatic effect is not expected. The opposed conductive layers rather constitute a capacitor, and in some cases, the electric charge accumulated in this capacitor cannot find any leakage path. Among conventional sheets and molded articles of a complicated profile used for the purpose of preventing electrostatic injury of electric products having integrated circuits built therein, none of them are satisfactory in antistatic effect and cost.